Cyanine dyes play an indispensable role in biomedical applications, particularly in fluorescence detection of antibodies and DNA, the imaging of biological targets in vivo, and fluorescent labeling compounds for proteins. This is due to their excellent spectral properties, including large molar extinction coefficients and broad wavelength tunabilities. The labeling of cyanine dyes to biomolecules often involves covalent conjugations in aqueous buffer solutions under mild conditions. However, planarity of the cyanine π system leads to aggregations and dimer formations in aqueous solution. Due to insufficient hydrophilicity, non-specific interactions with various surfaces have been observed, resulting in problems with purifications of the corresponding cyanine-labeled bioconjugations, and an unsatisfactory signal to noise ratio.
Functionalized polyethylene glycol (PEG) linkers with reactive groups are important building blocks with a wide range of applications in biochemical and pharmaceutical industries. PEG is non-toxic, non-immunogenic, non-antigenic, and highly soluble in water. Incorporations of functionalized PEG linkers into cyanine dyes not only improve dye water solubility, but also introduce functionalities in cyanine dyes for further conjugation with biomolecules.